The present invention relates generally to vehicles and machines that experience shifting centers of mass during use and, more particularly, to an apparatus and method for improving the stability of such vehicles.
To provide favorable ride, handling and traction, most automobiles and similar vehicles have their lateral centers of mass located at some point substantially along the longitudinal axes thereof. (See FIGS. 1A and 1B.) However, other vehicles, such as excavators, boom lifts and other machines, have centers of mass that move significantly during use. (See FIGS. 2A-2C.)
For example, as the boom of a boom lift is extended and a load is applied to the platform or bucket thereof, the lift's center of mass moves outwardly toward the supporting wheels or tracks. (See FIG. 3.) If a sufficient load is applied to the boom, the center of mass will move beyond the wheels and the lift will tip over. The imaginary line along a support surface (e.g., the ground) about which a vehicle tips has been denominated the "tipline."
By defining the tipline of a vehicle as near to the perimeter of the vehicle's chassis as possible, the stability of the vehicle is increased. This increase in stability permits the vehicle to perform its intended function with the minimum amount of necessary counterbalance weight, which results in lower costs, improved flotation on soft surfaces, easier transport, etc.
A standard vehicle has two lateral tiplines separated by a distance substantially equal to the track width thereof. Vehicles incorporating conventional tires tend to tip about a line defined substantially at the lateral centerline of the tires (i.e., where the tires contact the support surface). (See FIGS. 4A and 4B.)
To achieve maximum vehicle stability, narrow and stiff solid rubber tires are preferred on excavators and other similar machines. However, wide and soft pneumatic tires have typically been used instead to decrease the ground bearing pressure of narrow tires (i.e., to prevent damage to fragile surfaces or to improve vehicle flotation on soft surfaces), to allow the use of small diameter tires to provide clearance for rotating machine components while still providing adequate tire capacity, and to improve ride, comfort and traction.
In an attempt to balance the conflicting needs of vehicle stability on one hand and ride, comfort, flotation, load capacity and traction on the other hand, foam-filled and dual tires have been utilized.
Particularly in Europe, foam-filled tires are popular and widely used. These tires are typically formed by filling a standard pneumatic tire with a urethane foam. In the event of tire puncture, the urethane foam prevents the tire from deflating. Although foam-filled tires are not as costly as solid rubber tires, they possess all of the other disadvantages of solid rubber tires. Moreover, because the width and shape of foam-filled tires are the same as the original, pre-filled pneumatic tires, foam-filled tires do not provide as much vehicle stability as solid rubber tires.
As stated above, dual tires have also been used to improve vehicle stability. By employing dual tires, the tipline of a vehicle or machine is moved to the center of the outboard tire. (See FIG. 5.)
However, there is an inherent risk factor in using dual tires. If one tire of the pair is damaged during vehicle use, the other tire may be unintentionally overloaded. For example, if the outboard tire of a dual pair is deflated, the tipline of the vehicle is then defined at substantially the centerline of the inboard tire, which results in a less stable condition then if the vehicle were initially designed for only a single tire. Further, the use of dual tires results in the increased cost of installing and maintaining twice as many tires per vehicle.
It is worthwhile noting that two other devices, which have not been used to improve vehicle stability, have been developed for allowing vehicles to be operated on flat tires.
An external disk, which has a slightly smaller diameter than the respective tire of a vehicle, is mounted adjacent to and either inboard or outboard of the tire. If the tire is deflated, the disk supports the vehicle above the ground and allows it to be operated for a limited time (e.g., for emergencies) at restricted speeds. Such an external disk is disclosed in South African Patent No. 90/2717 and has been manufactured and sold in the Republic of South Africa by "Express Trailer Repairs" under the name "TRUCKSAVER."
Further, Hutchinson and Rodgard Corporation have marketed an internal disk that allows deflated pneumatic tires to operate for a limited time at restricted speeds and/or distances. The disk is mounted to a tire's rim at the centerline thereof, and is sized to contact the inside of the tire when the tire deflates to approximately 50% of its normal height. This device does not increase the tipping stability of the vehicle because the disk, and thus the tipline, is still positioned at substantially the centerline of the tire and a large deflection (i.e., greater than 50% of the tire's height) is needed for the internal disk to contact the tire and thus the ground.